1. Field of the Invention
The present invention relates to forced-air burners, and more particularly, but not by way of limitation, to the use of a modulating air damper to precisely control the combustion air flow rate to the fire box of the burner to produce high turn down ratios.
2. Background
Commercial and industrial burners are essentially of two types: atmospheric burners, in which the air required for combustion is attained without any prime mover (such as a blower or fan), and forced-air burners, which rely on a blower for feeding combustion air. Atmospheric burners are mainly limited to low heating rate domestic equipment, whereas forced-air burners comprise the majority of burners used in commercial and industrial applications.
Air regulation in forced-air burners can be achieved by installing louver boxes on air dampers, either at the blower inlet or outlet. Louver boxes are generally provided with a circular or rectangular configuration. The number and size of louvers in commercial burners can vary from one louver to several louvers, with two or three louvers being most common. In large industrial burners, as many as 10 to 15 louvers are not uncommon.
Louver actuating mechanisms typically provide either a parallel louver construction, wherein all blades open in the same direction, or an opposed louver construction, wherein adjacent louvers open in opposite directions. An opposed blade louver box is comparatively expensive to make but is superior to a same size parallel blade louver box, as an opposed blade louver box provides comparatively superior air flow modulation. Superior air modulation is critical to the obtaining of a high turn down ratio--the ratio of the high and low heating rates between which a burner can be safely modulated without exceeding emission standards. There is a long felt need in the industry for an improved way to precisely modulate combustion air to a burner, especially in regard to better controlling the air flow at low heating rates so as to increase the burner turn down ratio.
One recognized solution is to keep the air damper closed at low heating rates and allow the air that leaks through the damper to support combustion. This approach is taught in U.S. Pat. No. 3,869,243 issued to Creuz. While such an approach certainly allows the operation at lower heating rates, and thereby increasing the turn down ratio, the approach is unsettling to experienced boiler operators who would prefer to be in precise control of air flow modulation. The approach will also be unpredictable, in that manufacturing variances will determine the amount of air leakage that a closed damper will provide, and part to part variation will result in wide variation to low heating burner performance.
One skilled in the art will recognize the benefits of a precisely modulating air damper, having an opposed louver construction, and having louver seals which prevent air flow through the damper at a closed position, thereby enabling the boiler operator to finely trim air flow at low heating rates, resulting in improved burner turn down ratios.